1. Field of the Invention
The present invention relates to an electric generating system for automobiles and its control method, more particularly to an electric generator fit for a 4-wheel drive automobile which electrically drives wheels which are not driven directly by the internal combustion engine.
2. Related Background Art
As a conventional art of means which drives wheels which are not driven directly by the internal combustion engine, Japanese Non-examined Patent Publications H07-151514 (1995) discloses a means which runs an electric motor by a combination of a 14-V alternator and a 12-V battery for an auxiliary unit only on a low μ road which requires a starting assistant. Japanese Non-examined Patent Publications H07-231508 (1995) discloses a system which drives an electric motor by a combination of a 14-V alternator and a 12-V battery for auxiliary units.
Further Japanese Non-examined Utility Model Publications S55-110328 (1980) discloses a 4-wheel drive automobile which drives four wheels only when the running vehicle slips. In other words, this publication discloses an automobile driving either front wheels or rear wheels by a power of the engine, wherein said automobile comprises an auxiliary driving source (an electric motor and a generator) which drives the rear or front wheels which are not driven directly by the engine and a controller which detects a difference between front wheel revolutions and rear wheel revolutions and controls said auxiliary driving source.
In a low-voltage power generating system having a 14-V generator and a 12-V battery for an auxiliary unit, a large current is required to cause the electric motor to output enough power to get the full performance of the 4-wheel drive automobile. In other words, such a low-voltage power generating system requires a large current because a control voltage is low. Consequently, system components such as switches, electric circuits, and harnesses are required to withstand large currents. This necessarily increases production cost and component sizes.
Contrarily, if a driving torque is set low, the system cannot fully assist the 4-wheel drive automobile. Further, this low-voltage and large-current power applied to the whole electric circuit generates heat and reduces the efficiency.
Furthermore, as the size of the 12-V battery (for normal automobile electric loads) is limited, the electric energy output from the battery is low and the service life of the battery for the 4-wheel drive automobile becomes much limited. Therefore, this battery is not enough to go up a long continuous slope which requires high power for a long time.
If one generator is used both to charge the battery for the auxiliary unit and to drive the electric motor, various problems occur such as cost increase due to provision of the change-over switch, requirement of reliability, reduction in efficiency because different required output characteristics must be satisfied, increase of electric losses caused by said large current, and reduction of available energy.
If a driving circuit contains a voltage source such as a battery, its voltage value is automatically set by the characteristics of the voltage source and an optimum voltage required by the motor cannot be supplied.
Further, the system disclosed by Japanese Non-examined Utility Model Publications S55-110328 (1980) always makes a difference between front and rear wheel revolutions. This difference is dependent upon the “μ” value of the running road. Therefore, this system controls the output of the generator of the auxiliary driving source so as to follow the revolutions of the wheels driven by the engine. This system causes the automobile to work as a 4-wheel drive automobile only when the automobile slips, but usually, this system leaves the automobile as a 2-wheel drive automobile.